This invention relates to the reduction of noise in optical communications networks, and in particular, to the reduction of noise in an add path where signals are placed onto the network at a network node.
Add/Drop multiplexers are widely used in optical communications networks to provide wavelength non-specific add/drop ports. This allows random transponder provisioning and tuneability. FIG. 1 shows an example of how add/drop multiplexers are used. The optical network is a dual fibre DWDM optical network with one fibre 10 carrying traffic in an East/West direction and the other fibre 12 in a West/East direction. The terms East and West are conveniently used to describe the direction in which network traffic travels and does not correspond to geographical East or West. The traffic is a multiplex, typically, of 32 wavelength channels. At network nodes, traffic is taken off the network and split into the component channels using an optical demultiplexer or a splitter and band pass filter and added into the network using an optical multiplexer and an add coupler.
The network nodes can add or drop traffic to either of the network fibres. It is desirable to route traffic the shortest distance around the network which will depend on the location of the destination node. It is also desirable to have a fall back path should one of the E/W or W/E paths fail.
Thus, in FIG. 1, each of the E/W and W/E network fibres comprise an amplification stage 14. This is a EDFA amplifier although this may be omitted in shorter networks. The applied signal is split in a 2:1 coupler 16 to provide two signal output paths. A through path 18 carries traffic that remains on the network and a drop path 20 drops the signal multiplex from the network for processing at the network node.
Each of the two dropped paths are input to an optical demultiplexer 22 which splits the signal into its constituent wavelength components. The receive transponder includes a switch 24 to select the signal output from one of the two demultiplexers. In practice in a 32 channel node, this switch will receive 32 channels from each of the demultiplexers.
Signals remaining on the through path pass through a channel control unit 26 and then to an add coupler 28 in which signals from the transmit side of the node transponder are added onto the network. The output of the add coupler is finally amplified again at 30 if required.
The add side of the transponder comprises an amplifier 32 and an n:1 add coupler 34, where n is the number of wavelengths in the multiplex carried by the network. The add coupler is required to add the individual wavelength signals that are to be placed onto the network. Use of add couplers has the disadvantage of incurring losses and causing the add power level to require amplification. Thus, the add signal is amplified. This in turn causes broadband noise which passes into the system degrading the OSNR (optical signal to noise ratio) of the added signals as well as those signals passing through the photonic add/drop node. It is known to add tuneable filters 36 following the add amplifier to remove the noise added to the through channels.
In the figure, the add coupler is shown as a multiplexer 34. Separate multiplexers may be used on the east and west paths.
The noise that is generated is dependant on the gain of the amplifier in the add path. This gain is determined by the maximum possible losses in the add path. When a signal is added, the source powers are adjusted on a per channel basis to achieve the required power at the point of addition. The signal to noise ratio is therefore worst when the loss of the add path is less than the maximum loss possible. The OSNR is optimised when the path loss is a maximum as the noise from the EDFA amplifier experiences the maximum loss.
This situation is undesirable and the present invention seeks to overcome by improving or optimising the add channel OSNR.